winsys/radeon: fix a race condition between winsys_create and winsys_destroy

This also hides the reference count from drivers.

v2: update the reference count while the mutex is locked in winsys_create

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
6 files changed